1. Field of the Invention
The present disclosure relates to doping a surface of a thin film or the interface between two thin films by atomic layer deposition. In particular, the disclosure concerns atomic layer deposition methods that utilize a blocking reactant to provide for uniform doping of a substrate at a desired level.
2. Description of the Related Art
In semiconductor device applications, dopants are often intentionally introduced at various locations of the device structure in order to modulate device performance. Area density of the dopant is important to achieve the desired effect and performance. In the past, element addition, or doping, was customarily achieved by utilizing conventional thermal diffusion in furnace, ion-implantation and Chemical or Physical Vapor Deposition (CVD, PVD), such as sputtering. It is extremely difficult, however, to achieve uniform doping across a large substrate due to dose variation. In addition, it is difficult to control the doping profile. Atomic Layer Deposition (ALD) processes, as described herein, are a practical alternative to uniformly controlling doping across a large substrate, and allow for control of the area density of dopant at desired locations.
Traditional ALD is a self-limiting process, whereby alternated pulses of reaction precursors saturate a substrate surface and leave no more than one monolayer of material per pulse. The deposition conditions and precursors are typically selected to ensure self-saturating reactions, such that an adsorbed layer in one pulse leaves a surface termination that is non-reactive with the additional gas phase reactants of the same pulse. A subsequent pulse of different reactants reacts with the previous termination to enable continued deposition. Thus each cycle of alternated pulses leaves no more than about one molecular layer of the desired material. The principles of ALD type processes have been presented, for example, by T. Suntola, e.g. in the Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601-663, Elsevier Science B.V. 1994, the disclosure of which is incorporated herein by reference.
In a typical ALD process for depositing thin films, one deposition cycle comprises exposing the substrate to a first reactant, removing unreacted first reactant and reaction byproducts from the reaction chamber, exposing the substrate to a second reactant, followed by a second removal step.
In previous ALD methods that could be used for doping, dopant uniformity could be obtained through saturating binding sites on the surface with a dopant precursor reactant. However, dopant levels would thus be determined by the number of available binding sites. While a lower dopant concentration could theoretically be obtained through a non-saturated doping reaction by using a limited amount of precursor, such a process would lead to non-uniformity of dopant across the substrate, particularly in a cross flow type reactor.